As a radiation detector for measuring a dosage based on radiation, a detector such as an ionization chamber, a GM (Geiger-Muller) counter tube, a scintillation detector or a semiconductor detector is widely used as a dosage monitor such as an area monitor, a process monitor or a survey meter. A radiophotoluminescence glass dosimeter, a thermoluminescence dosimeter, or an optically stimulated luminescent dosimeter is used as an individual radiation exposure management or imaging plate. These dosage monitor, individual dosimeter and imaging plate are used mainly in atomic energy facilities, medical facilities using radioactive isotope, or research accelerator facilities.
In a conventional method using a dosage monitor, it is general that an application voltage of several volts to thousands of volts is supplied to the aforementioned detector to operate the detector an electrical signal associated with radiation is sent from the detector via a coaxial cable or the like to a measurement device provided at a downstream stage to analyze the signal. As an example, the detector applied to the dosage monitor used in atomic energy facilities is one of a silicon semiconductor detector, an NaI (TI) scintillation detector and an ionization chamber, any of which requires electricity supply thereto.
In the aforementioned various facilities, dosage monitoring based on these detectors can be achieved without any troubles in a usual operating situation. However, when power supply cannot be achieved due to a power failure or the like, it becomes difficult to measure a dosage.
As a radiation measurement means which eliminates the need of power supply to the detector, a dosage monitor using optical fiber is suggested. The arrangement is that scintillation element is connected at a tip end of an optical fiber, light applied to the element is converted to an electrical signal by an optical detector provided at a downstream stage of the fiber for analysis thereof. As a position measurement means, an optical detector is provided at each of both ends of an optical fiber and a detection position is derived by using a time difference between light detections of the detectors.
However, even in any of the above systems, the amount of scintillation light is very small and it is difficult to prolong an optical fiber length from the viewpoint of SN ratio. Accordingly, such a system cannot be applied to such atomic energy facilities as to require long wiring of the cables. In order to achieve dosage monitoring using optical fiber in such facilities, a measurement system and a measurement method for reducing attenuation due to optical fiber length and SN degradation due to background are required.
Typical fluorescent materials for converting a dosage to a light quantity include NaI (TI), BGO, and GSO, and light irradiated immediately after occurrence of an interaction with radiation is used to analyze a dosage, a radiation energy and so on. An optically stimulated luminescent (OSL) element, utilizing a proportional relationship between the formation amount of a color center and an accumulated dosage, analyzes an accumulated dosage from OSL light irradiated when optically stimulated light from a light source is transited from an excited state to a base state.
PATENT LITERATURE 1 is characterized in that an OSL element is connected at a tip end of an optical fiber, stimulated light is irradiated from a laser light source thereto to thereby generate scintillation light originated from the OSL, and the light is measured by an optical detector provided at a stage downstream thereof.
PATENT LITERATURE 2 is characterized in that a device for analyzing the fluorescent dosage of a fluorescent material contained in a sample includes a means for increasing a multiplication factor of an optical detector only in a time range where the fluorescent dosage of a fluorescent material is dominant on the basis of a pulse light generation timing signal of a light source.